Analysis of the retrogradation of low starch concentration gels using differential scanning calorimetry, rheology, and nuclear magnetic resonance spectroscopy

The retrogradation of 5, 10, 15, and 25% corn starch gels was measured using differential scanning calorimetry (DSC), rheology, and an array of NMR spectroscopy techniques. During the initial (<24 h) stage of retrogradation, an increase in G′ corresponding to an increase in the number of solid protons participating in cross-relaxation (M_O^B) was observed for all four concentrations studied. During the latter (>24 h) stage of retrogradation, amylopectin recrystallization becomes the dominant process as measured by an increase in ΔH_r for the 25% starch gel, which corresponded to a further increase in M_O^B. A decrease in the molecular mobility of the liquid component was observed by decreases in ¹⁷O T₂, ¹H D_O, and T_2A. The value for T_2B (the solid transverse relaxation time) did not change with concentration or time indicating that the mobility of the solid component does not change over time despite the conversion of the highly mobile starch fraction to the less mobile solid state during retrogradation.